1. Field of the Invention
The present invention relates to an arc tube and a method for manufacturing an arc tube, and more particularly to an arc tube and a method for manufacturing an arc tube that can be used as a light source for a headlamp of a vehicle.
2. Description of the Related Art
In recent years, an arc tube has often been used as a light source of a headlamp for a vehicle because it can carry out irradiation with a high luminance. As shown in FIG. 12, an arc tube to be used in a headlamp for a vehicle generally has an arc tube body 104 formed of a glass material in which a pinch seal portion 104b is provided on both sides of a light emitting tube portion 104a forming a discharge space 102. The arc tube includes a pair of electrode assemblies 106, each having a tungsten electrode 108 and a lead wire 110 coupled and fixed to each other through a molybdenum foil 112. Each electrode assembly 106 is pinch sealed with the arc tube body 104 in each pinch seal portion 104b. By the pinch seal, the molybdenum foil 112 is joined with the arc tube body 104 in such a state as to be embedded in the arc tube body 104.
In a conventional arc tube as shown in FIG. 12, however, the junction strength of the molybdenum foil 112 and the arc tube body 104 is not sufficient. For this reason, the molybdenum foil 112 is easily peeled in the junction surface of the molybdenum foil 112 and the arc tube body 104 during the use of the arc tube. When such peeling is caused, a crack is generated on the arc tube body 104 from the edge of the junction surface and grows to finally generate a leakage between the discharge space 102 and an external space. Accordingly, the lifetime of a conventional arc tube is comparatively short.
Also in the conventional arc tube, a slight compressive stress remains at an ordinary temperature along the junction surface of the arc tube body and the molybdenum foil (a tensile stress remains in the molybdenum foil), and the coefficient of linear expansion of the molybdenum foil is much greater than (approximately 10 times as great as) that of the arc tube body. Therefore, when the temperature is raised by turning on the arc tube, tensile stress is generated on the arc tube body (the compressive stress is generated on the molybdenum foil). For this reason, the compressive stress and the tensile stress are alternately generated on the arc tube body by repeatedly turning on and off the arc tube. Consequently, the engagement state of the molybdenum foil and the arc tube body is broken so that the molybdenum foil easily peels.